MONDAY, March 9, 2015 (HealthDay News) -- Hard physical work, high blood pressure and taking multiple medications are among the factors that may lower sperm quality and make men less fertile, new research finds.

"Nearly 15 percent of American couples do not get pregnant within their first year of trying," and male infertility plays a major role, study senior author Germaine Buck Louis, director of the division of intramural population health research at the U.S. National Institute of Child Health and Human Development, said in an agency news release.

Her team looked at 456 men in Texas and Michigan, average age 32, who were in committed relationships and had stopped using contraception. Most of the men (77 percent) were white and more than half had never made a woman pregnant.

Semen analysis revealed that 13 percent of the men who had physically demanding jobs had low sperm counts, compared to 6 percent of those who didn't exert themselves at work. No other work-related factors -- such as heat, noise or prolonged sitting -- appeared to affect semen quality.

Men who had been diagnosed with high blood pressure also had a lower percentage of normally shaped sperm than those without high blood pressure, the findings showed.

The study could only point to associations between various factors and lowered fertility, it could not prove cause-and-effect.

Still, "as men are having children later in life, the importance of diseases we once thought as separate from fertility must be re-explored," said study principal investigator Dr. Michael Eisenberg, director of male reproductive medicine and surgery at Stanford University in California.

"Future investigations need to examine whether it's the high blood pressure itself or the treatment that is driving these trends," he said in the news release.

The researchers also found that men who took multiple medications were more likely to have low sperm counts. A normal sperm count is between 40 million and 300 million. Fifteen percent of men who took two or more medications had sperm counts below 39 million, compared with 7 percent of those who took no medications.

"The good news is that these factors, if they are confirmed to have negative effects on male fertility, can potentially be modified by medical care or changing job-related behaviors," Buck Louis, of the National Institutes of Health, said in the news release.

Two fertility experts agreed, saying the new study could offer men some guidance.

"If you are a man experiencing trouble conceiving, it would be beneficial to take occupational hazards and personal health into consideration," said Dr. David Samadi, chair of urology at Lenox Hill Hospital in New York City. He believes that "making a few lifestyle changes to battle high blood pressure, or reducing the amount of heavy lifting at work could go a long way on the road to fatherhood."

Dr. Doron Stember is assistant professor of urology at the Icahn School of Medicine at Mount Sinai in New York City. He said that "diabetes, obesity and the metabolic syndrome have previously been associated with poor semen quality," and now high blood pressure can be added to that list.

"The key message is that lifestyle factors, such as activity and diet, can play an important role for men seeking fertility," Stember said.

The study was published online March 9 in the journal Fertility and Sterility.